Many chokes are known for controlling a flow rate of a fluid in a fluid passage. Typically, these consist of orifices or venturis placed in the passage to reduce the flow which results from a given pressure drop across the choke. Fluid flow systems often have problems involving contamination by particulate contamination or other debris which may become lodged in the choke passageway, and change the relationship between the pressure drop across the choke, and the flow rate through it. Generally, the effect of debris is to decrease the flow rate at a given pressure drop.
The specific application for which the present invention is intended is as a precision choke for exhausting air from an enclosed volume in a railroad brake air valve at a precisely controlled rate.
The prior art choke for this application is formed as a plug having a pipe thread on its exterior for assembly into an exhaust port in a casting of the brake valve. The device has a constriction, and it has a felt plug upstream of the constriction to remove debris. This configuration has a disadvantage in that the felt plug introduces a pressure drop which is difficult to control. Hence, this prior art choke provides only an imprecise choking action. That is, the flow rate for a given pressure drop is uncertain within a range of values due to uncertainties due to the packing of the plug, and variations in assembly of the felt plug to the choke. An additional disadvantage of the prior art choke is that the felt plug does not provide an absolute barrier against contaminant particles above a predetermined size, due to the random disposition of fibers in the felt.